The present invention deals with a process for the manufacture of cellulosic moulded bodies, in particular cellulosic flat films and cellulosic membranes in the form of flat membranes whereby a solution of cellulose in an aqueous tertiary amine oxide is moulded in film form by means of an extrusion die, which has an oblong extrusion gap, and led through an air gap into a precipitation bath whereby the cellulosic flat film is formed in the precipitation bath.
From U.S. Pat. No. 2,179,181 it is known that tertiary amine oxides have the ability to dissolve cellulose and that cellulosic moulded bodies such as fibres can be won from these solutions as a result of precipitation/regeneration. A process for the production of solutions of this kind is for example known from EP-A-0 356 419. According to this publication first of all a suspension of cellulose is prepared in an aqueous tertiary amine oxide. The amine oxide contains up to 40 weight % water. The aqueous cellulose suspension is transferred into the solution in a thin film treatment apparatus.
From DE-A-28 44 163 it is known for the production of cellulose fibres that an air gap is provided between the spinning nozzle and the precipitation bath to achieve drawing at the nozzle. This nozzle drawing is necessary since the stretching of the filaments is made more difficult after the contact of the moulded spinning solution with the aqueous precipitation bath. In the precipitation bath the fibre structure set in the air gap is fixed.
A process for the production of cellulosic threads is, furthermore, known from DE-A-28 30 685 whereby a solution of cellulose is formed to filaments in a tertiary amine oxide in a warm condition, the filaments are cooled down with air and finally introduced to a precipitation bath to precipitate the dissolved cellulose. The surface of the spun filaments is, furthermore, moistened with water to reduce their tendency to stick to neighboring filaments.
From DE-A-195 15 137 a process is known for the production of flat films whereby first of all a tubular film is formed using a ring nozzle, said film being cut to flat films following washing and drying. When manufacturing the tubular films, the extruded tube is extended in the air gap both in the drafting direction and in the transverse direction. This happens as a result of gas pressure effective in the inside of the tube. The disadvantages of this process lie in the complicated design of the device to be used and in the washing and drying process which is more complex with tubular films than with flat films.
Processes for the manufacture of cellulosic tubular films are, moreover, well known from U.S. Pat. No. 5,277,857 and EP-A-0 662 283. According to these known processes a cellulose solution is formed to a tube via an extrusion nozzle with a ring-shaped extrusion gap, said tube being drawn over a cylindrical mandrel and introduced to a precipitation bath. In order that the extruded tube does not stick to the surface of the mandrel its surface is covered with a water film so that the inner side of the tube coagulates and slides over the cylindrical mandrel. According to EP-A-0 662 283 the tubular film is extended after washing by blowing in a gas.
DE-C-44 21482 describes a blowing process for the manufacture of oriented cellulosic films whereby the cellulose solution is extruded via a film blowing nozzle and an air gap downwards into a precipitation bath. It is mentioned that stretching can be performed transverse to the transport direction of the blown film via the gas pressure in the inside of the blown film and that the relation of mechanical longitudinal and transverse properties can be set.
A process and a device for the manufacture of cellulosic films, particularly of tubular films, is also known from WO-A-95/07811 of the applicant. In this respect the dissolved cellulose is cooled before it is brought into the precipitation bath by subjecting the heated solution to a stream of gas immediately after extruding.
From WO-A-97/24215 there is known a process for the production of an oriented cellulosic film in which a cellulosic solution is applied to a stretchable surface to which the solution sticks, the solution is then stretched by stretching the extendable surface and finally it is precipitated.
From EP-B-0 494 851 of the patent applicant there is known a process for the production of a cellulosic flat film in which a cellulosic solution is pressed through a nozzle or gap, it is then led through an air gap and then coagulated in a precipitation bath and the coagulated flat film is stretched in the longitudinal direction.
With cellulosic membranes particularly in the form of flat membranes, i.e. membranes from a flat film, the permeability of the membranes is an important property. To solve certain separation tasks it is important to select membranes with the optimum permeability, pore size and pore structure for the respective separation task.
Dialysis membranes made of regenerated cellulose in the form of flat films, tubular films or hollow threads have been known for some time whereby the regeneration of the cellulose can take place by means of the cuoxam-process, the viscose process or by means of the hydrolysis of cellulose acetate. Depending upon the process used and the process conditions one obtains membranes with different dialysis properties.
U.S. Pat. No. 4,354,938 describes for example a process for the production of dialysis membranes according to the viscose process, in which a tubular moulded membrane is stretched in the transverse direction by between 40 and 120% by blowing up with air before drying, which leads to a membrane with a regular orientation in the longitudinal and transverse direction. When transferring the dried membranes in the wet state the membranes thus produced undergo a shrinkage in the longitudinal and transverse direction of 0.5-10%. The ultrafiltration values lie in the range of between 2.5 ml/m.sup.2.h.mm Hg and 5.2 ml/m.sup.2.h.mm Hg at a wet thickness of 184 .mu.m to 45 .mu.m.
In "membranes and membrane processes" by E. Staude, 1992, VCH Verlagsges.m.b.H. on page 19 it is described that the biaxial stretching of finished cellophane membranes leads to the enlargement of the pores, monoaxial stretching on the other hand leads to a reduction in the effective pore diameter.
However, the viscose process only offers limited possibilities to set membrane properties in a well aimed manner. Moreover, the recovery of the chemicals which accumulate in this process such as sodium sulphate and carbon bisulphide etc. is very expensive.